1. Field of the Invention
The present invention relates to a semiconductor IC package containing an IC chip comprising semiconductor devices and, more specifically, to an outer lead, i.e., external terminals, provided on a ceramic package to connect the semiconductor devices contained in the ceramic package electrically to external electric circuits, and a method of fabricating such an outer lead.
2. Description of the Related Art
There are various known packages including those of a flat package type and a pin grid array type for containing a semiconductor IC comprising semiconductor devices. These packages are provided with leads to connect semiconductor devices contained therein electrically to external electric circuits. Generally, when fabricating the outer lead, the surface of a base plate is subjected to Ni plating for surface preparation, and then the Ni-plated surface is subjected to Au plating to form an Au surface layer for preventing the deterioration of the surface quality of the outer lead attributable to corrosion or oxidation and to secure satisfactory conductivity and soldering property.
The base plate thus coated with the plated layers is still subject to corrosion. Although the corrosion of the plated metal layers, the peeling of the incompletely plated metal layers and such are possible causes of corrosion of the base plate, the dissolution of part of the base plate into water penetrated through pinholes penetrating the plated metal layers and reaching the surface of the base plate and acting as an electrolyte that provides local cells resulting from the difference in potential between the plated metal layer and the base plate is the most general cause of corrosion of the base plate.
If the base plate dissolves reducing its sectional area, there is the possibility that the strength of the base plate becomes reduced and, in the worst case, the base plate breaks. A prevalent means for preventing the corrosion of the base plate merely forms a plurality of superposed plated layers to reduce pinholes penetrating through the plated layers and reaching the surface of the base plate. Other means for preventing the corrosion of the base plate forms a plurality of plated layers of different materials or finishes the base plate or each plated layer by washing.
For example, a four-layer electric contact material is disclosed in Japanese Patent Laid-open (Kokai) No. Hei 4-294009. This electric contact material consists of a Cu or Cu alloy base plate, i.e., a first layer, an amorphous alloy or Co alloy layer, i.e., a second layer, formed over the surface of the base plate to prevent the corrosion and diffusion of the material of the base plate, a Ni alloy layer, i.e., a third layer, formed over the amorphous alloy or the Co alloy layer, and a corrosion-resistant plated Au layer, i.e., a fourth layer, formed over the Ni alloy layer. The Ni alloy layer is formed to reduce the surface potential difference between the Au layer and the layer underlying the Au layer. This electric contact material is featured by the plurality of plated layers of different materials, and is intended to prevent the corrosion of the Cu or Cu alloy base plate by suppressing the formation of a local cell by the third and the fourth layer providing a small potential difference therebetween.
However, the conventional outer lead having a plurality of superposed plated layers of different materials to prevent the corrosion of the base or merely incorporating means for preventing the formation of a local cell has the following disadvantages. Although the plurality of simply superposed plated layers are effective in reducing pinholes penetrating the plated layers and reaching the surface of the base plate, the plurality of plated layers increases the size of the leads of the outer lead and such leads having an increased size can be connected only to a special external electric circuit. If the thickness of each of the plated layers is reduced, pinholes reaching the surface of the base plate cannot be reduced. On the other hand, excessively thick plated layers and the plurality of plated layers will increase the cost of the outer lead.
The recent development of multifunction ICs and progressive increase in the degree of integration of the component devices ICs require an outer lead having a large number of leads arranged at very small intervals. In such an outer lead, external leads coated with a plurality of plated layers and having an increased size are liable to be short-circuited. Although the external leads of a conventional outer lead, having a comparatively large sectional area do not break even if its base plate is corroded due to the corrosive action of water penetrating the plated layers through pinholes formed in the plated layers, very small leads, such as leads of 0.18 mm in width and 0.13 mm in thickness, breaks even if only a small portion of the base plate is corroded. If even a single lead among those of the outer lead has pinholes reaching the surface of the base plate, the lead having pinholes is corroded eventually, causing the electronic apparatus provided with this outer lead to malfunction. Therefore, the entire surface of the base plate of the outer lead must be coated with plated layers perfectly free of pinholes reaching the base plate. However, it cannot be expected that such plated layers perfectly free of pinholes reaching the base plate can be formed by a plating process which simply superposes a plurality of plated layers, and the plating process has a low yield rate and hence is practically applicable.
An attempt to enhance the corrosion resistance of the outer lead by providing the outer lead with a plurality of plated layers of different materials requires a plurality of plating baths, which increases the cost of equipment, and a plurality of plating processes, which complicates the outer lead fabricating process.